Threaded connections are in widespread use as connection for connecting the oil country tubular goods used for exploration of natural gas fields, oil fields, and the like located at depths of several thousands meters, and for exploitation of deposits therefrom. There are two types of threaded connections for the oil country tubular goods. One is an integral type of threaded connection provided with a pin at one end of a pipe, and a box at the other end of the pipe so that respective ends of two pipes are connected with each other by screwing the pin into the box. The other is a coupling type of threaded connection for connecting respective ends of two pipes with each other, each pipe provided with a pin at both ends, by means of a coupling provided with two boxes. The followings are performance required of these threaded connections in common:
ability to withstand axial tensile force caused by weights of connected pipes themselves PA1 ability to withstand internal pressure caused by a fluid flowing inside the pipes, and external pressure caused by fluids flowing outside the pipes PA1 durability to enable repeated uses as frequent scores of times PA1 ability to resist corrosion and erosion caused by fluids flowing inside and outside the pipes PA1 (1) The seal forming faces on the pin and box, respectively, are rotatable faces, and the pitch diameter of the seal forming face 13 on the pin prior to making up the threaded connection is larger than a pitch diameter of the seal forming face 23 on the box prior to making up the threaded connection; and PA1 (2) An inclination (.theta.1) of the torque shoulder forming face on the pin prior to making up the threaded connection is smaller than an inclination (.theta.2) of the torque shoulder forming face on the box prior to making up the threaded connection, the difference (.DELTA..theta.) between the both inclinations being in the range of 0.5 to 4.0.degree.. PA1 (3) An inclination (.theta.1) of the torque shoulder forming face on the pin, prior to making up the threaded connection is larger than an inclination (.theta.2) of the torque shoulder forming face on the box prior to making up the threaded connection, the difference (.DELTA..theta.) between both inclinations being in the range of 0.5 to 4.0.degree.. PA1 (4) Regarding to the relation between the inclination (.theta.1) of the torque shoulder forming face on the pin, prior to making up the threaded connection, and the inclination (.theta.2) of the torque shoulder forming face on the box, prior to making up the threaded connection, .theta.1 is smaller than .theta.2 for the internal shoulder type and .theta.1 is larger than .theta.2 for the external shoulder type, the difference (.DELTA..theta.) between both inclinations being in the range of 0.5 to 4.0.degree.. PA1 (5) The difference (.DELTA..theta.) between the inclination (.theta.1) of the torque shoulder forming face on the pin prior to making up and the inclination (.theta.2) of the torque shoulder forming face on the box prior to making up is proportional to D/t, and .delta.r, respectively. In case the value of D/t or .delta.r is large, the difference in inclination (.DELTA..theta.) between the torque shoulder forming face on the pin and the same on the box is increased, and in case the value of D/t or .delta.r is small, the difference in inclination (.DELTA..theta.) between the torque shoulder forming face on the pin and the same on the box is reduced wherein
Lately, depths of oil wells have increased, and more often, oil is exploited from oil wells in severe environments, for example, in the polar regions. Consequently, in an increasing number of cases, the oil country tubular goods and the threaded connections are used in such severe environments. Furthermore, deposits which were not intended for exploitation before have since come to be exploited. These include, for example, a deposit which is corrosive to steel and contains significant amounts of oxidizing impurities. Accordingly, threaded connections capable of satisfying requirements described above in such severe environments are called for, and the performance of the threaded connections has since been improved at a high pace.
FIG. 1 is a schematic illustrating a coupling type threaded connection provided with a seal portion and a torque shoulder portion. In the figure, an example of the threaded connection for the oil country tubular goods provided with the torque shoulder portion internally is shown. FIG. 1 (a) is a view showing pipes body 10 and a coupling 20 making up the threaded connection. FIG. 1 (b) is a partially enlarged view of a threaded portion, the seal portion, and the torque shoulder portion, as shown in FIG. 1 (a).
The torque shoulder portion is provided to prevent build-up of such a high contact pressure as to cause plastic deformation of the seal portion. The threaded connection is constituted such that a make-up torque can be controlled to an appropriate value by contacting a torque shoulder forming face 24 on the coupling side against a torque shoulder forming face 14 on the pipe side. Such a type of threaded connection provided with the torque forming faces 14 and 24, close to the inner surface of a pipe, as described above or the torque shoulder portion is called an internal shoulder.
FIG. 2 is a schematic illustrating an example of a threaded connection for the oil country tubular goods, provided with both the internal torque shoulder and an external torque shoulder. FIG. 2 (a) is a sectional view showing the pipe body and the coupling after the threaded connection is made up. FIG. 2 (b) is a partially enlarged view of the torque shoulder forming portion on the outer side of the threaded connection shown in FIG. 2 (a), showing inclinations of the torque shoulder forming faces on a pin and a box, respectively, for forming an external torque shoulder. The external torque shoulder is formed when the torque shoulder forming face 54 disposed at the root of a male thread section on the outer surface of the pipe is contacted against the torque shoulder forming face 64 disposed at the tip on the box.
Among the coupling type threaded connections, provided with a seal portion and a torque shoulder portion, an internal shoulder type threaded connection is described hereafter.
In FIG. 1 (b), male threads 12 provided on the surface of a tip portion of the pipe 10 and female threads 22 provided on the inner surface of the coupling 20 are mated with each other. A seal forming face 13, which is a rotatable face, is provided at the tip of a pin 11. The seal forming face 13 has a curved surface, tapered in the axial direction of the pipe or substantially resembling a circular arc of a large curvature radius. A box 21 of the coupling 20 is provided with a seal forming face 23, which is a rotatable face, at the root of a threaded portion thereof, and the seal forming face 23 has a curved surface tapered in the axial direction of the pipe. The seal portion is formed when the seal forming face 13 of the pipe body is brought into contact with the seal forming face 23 of the coupling for making up the threaded connection.
Owing to metal-to-metal seal accomplished at the seal portion, gas tightness of the threaded connection is maintained against internal pressure caused by a fluid flowing inside the oil country tubular goods, and external pressure caused by fluids flowing outside same.
The torque shoulder portion is formed when a torque shoulder forming face 14 on the pin, disposed beyond the seal forming face 13 of the pipe body, closer to the tip, is contacted against a torque shoulder forming face 24 disposed on the inner side of the seal forming face 23 on the box of the coupling.
In the case of the threaded connection for the oil country tubular goods, corrosion resistance against crevice corrosion causing leakage of fluids or rupture of the threaded portion is called for in addition to seal tightness against fluids under high pressure as described in the foregoing. Crevice corrosion occurs when a corrosive fluid intrudes into minuscule crevices formed between pin members and box members, stagnates therein, and consequently are enriched. When the threaded connection is loaded with high stress and placed under the conditions described above, crevice corrosion such as general corrosion, stress corrosion cracking, or the like occurs inside the threaded connection. No crevice corrosion occurs, however, in case that there exists no minuscule crevice on either the inner surface or the outer surface side of the pipe in which a corrosive fluid exists.
Threaded connections provided with the seal portion and the torque shoulder in shapes described hereafter have been proposed as improvements on the threaded connection shown in FIG. 1 for enhancing crevice corrosion resistance. These threaded connections are broadly classified into four classes, that is, one provided with contact faces coated with a softer material, one using a packing, one having enhanced seal tightness of the seal portion, taking advantage of elastic deformation of the pin at the tip thereof, and one having enhanced gas tightness of the seal portion by means of a device for improved contact of the torque shoulder forming faces.
The threaded connections with contact faces coated with a softer material as disclosed in Japanese Patent Laid-open (Kokai) No. 56-109975, No. 1-199088, and the like have a drawback in that gas tightness thereof is impaired in the course of repeated making up and disconnection of the threaded connection due to exfoliation and wear-out of the coating material with the result that these threaded connections are unable to withstand repeated uses as frequent as scores of times.
In the case of the threaded connection using a packing as disclosed in Japanese Patent Laid-open (Kokai) No. 58-142475, same has a drawback of extra time and expenses required in production, fitting, and the like of the packing. Further, in the case of this type of threaded connection without a function for controlling a torque for make-up at an appropriate value, which is naturally required of the torque shoulder, strict torque control is required at a work site when making up connection. Consequently, operativity of the threaded connection is poor. In addition, same poses a problem of susceptibility to crevice corrosion because crevices may be formed between the packing and the torque shoulder depending on manufacturing tolerance.
In the case of the threaded connection having enhanced gas tightness, taking advantage of elastic deformation occurring at the tip of the pin (for example, as disclosed in Japanese Patent Laid-open (Kokai) No. 60-26878), substantial bending deformation needs to be caused to occur to a lip portion at the tip of the pin in order to obtain a seal face having a sufficient contact pressure in a seal forming portion. For this purpose, the lip portion at the tip of the pin needs to be pressed by a large force to the torque shoulder forming face on the box. As a result, excessive plastic deformation occurs on the torque shoulder forming face, particularly, at a portion in close vicinity of the pipe surface.
FIG. 3 shows an example of the threaded connection having enhanced gas tightness by devising improved contact on the torque shoulder forming faces (refer to Japanese Patent Laid-open (Kokai) No. 52-11767). With the threaded connection, swelling of the pin in close vicinity of the tip thereof is caused to occur in the direction of the outside diameter of the pipe through contact between the torque shoulder forming face on the pin and same on the box so that seal tightness of the seal portion is enhanced. More specifically, a seal forming face 13 on the pin is rendered a rotatable face having a surface in the shape of a circular arc of a large curvature radius in sectional view, and a seal forming face on the box is rendered a circular cone (a rotatable face having a surface of a tapered straight line in sectional view). Furthermore, a torque shoulder forming face 14 on the pin and a torque shoulder forming face 24 on the box are rendered to have a convex surface and a concave surface, respectively, each having a differentiated curvature radius, R2 and R3. In this connection, the curvature radius R2 of the torque shoulder forming face on the pin is rendered smaller than the curvature radius R3 of the torque shoulder forming face on the box, and in addition, the center of the curvature with the radius R2 is positioned on the inner side towards the pipe than that of the curvature with the radius R3. With adoption of such a constitution as described above, bending of the lip portion 15 on the pin towards the inner surface of the pipe is reduced, and even when nonuniform contact pressure is developed in the peripheral direction of the seal portion due to manufacturing tolerances, uniform contact pressure is automatically redistributed. It is claimed for this reason that satisfactory seal tightness is provided at two locations, namely, the torque shoulder portion and the seal portion.
In the threaded connection described, contact at the torque shoulder portion is linear contact along a circular face at portions 16 and 26 in close vicinity of the inner surface of the pipe. Consequently, contact pressure at such contact portions becomes very high, creating a problem of gas tightness being impaired due to excessive deformation, similarly to the case of the threaded connection as disclosed in Japanese Patent Laid-open (Kokai) No. 60-26878 described above.
The invention has been developed to solve the problems described above, encountered by the conventional threaded connections. Thus, it is an object of the invention to provide a threaded connection capable of preventing initiation of crevice corrosion without bringing about an increase in manufacturing cost thereof and deterioration in operativity, and also of demonstrating a high degree of gas tightness even after repeated uses as frequent as scores of times.